1. Field of the Invention
The present invention relates to gelatin hardening agents, particularly, to gelatin hardening agents used for photographic sensitive materials.
2. Description of the Prior Art
Gelatin is used as a binder for many photographic sensitive materials. For example, gelatin is used as a main ingredient for silver halide emulsion layers, emulsion protective layers, filter layers, intermediate layers, antihalation layers, backing layers, subbing layers for a film base or a baryta layer, etc.
Such sensitive materials containing gelatin are processed using various aqueous solutions each having a different pH or at different temperatures. However, layers containing gelatin which do not contain a hardening agent have poor resistance to processing and excessively swell in aqueous solutions whereby they are easily damaged. Particularly, in an aqueous solution of a temperature of 30.degree. C or more, gelatin layers sometimes dissolve and flow in some cases.
It is known that a number of compounds are effective to improve water resistance, heat resistance and scratch resistance of a gelatin layer. These compounds are known hardeners for photographic sensitive materials. For example, organic hardening agents such as aldehyde compounds such as formaldehyde or glutaraldehyde, etc., reactive halogen containing compounds as described in U.S. Pat. No. 3,288,775, compounds having a reactive ethylenically unsaturated bond as described in U.S. Pat. No. 3,635,718, aziridine compounds as described in U.S. Pat. No. 3,017,280, epoxy compounds as described in U.S. Pat. No. 3,091,537, halogenocarboxyaldehydes such as mucochloric acid, dioxanes such as dihydroxydioxane or dichlorodioxane, substituted methyl esters (for example, such as ##STR2## etc.) as described in U.S. Pat. Nos. 3,241,972 and 3,542,558 and Japanese Patent Publication 48,896/72, and inorganic hardening agents such as chromium alum, chromium sulfate, aluminium sulfate, potassium alum, ammonia alum or zirconium sulfate, etc., are well known.
However, these known gelatin hardening agents all have some defect when they are used in photographic sensitive materials; that is, there are those having an insufficient hardening function, those which cause changes of the hardening function by the lapse of time, those which cause "post-hardening" because of a slow hardening reaction with gelatin, those which have a harmful influence upon properties of the photographic sensitive materials (particularly, which increase fog, deteriorate sensitivity and change gradation, etc.), those which lose their hardening function due to the presence of other photographic additives or decrease effects of other photographic additives (for example, color couplers for color sensitive materials, etc.), those which are hard to produce in large quantities because synthesis is difficult, and those which have poor preservability beacuse of inherent instability.
Many hardening agents are unstable and have poor preservability because they are, in general, active compounds, and are unsuitable for mass production of the photographic sensitive materials having stabilized properties. Hardening agents having good preservability hitherto known, for example, inorganic hardening agents such as chromium alum, are unsuitable for practical use because they have a harmful influence upon the human body.